Internal combustion engines are well known and widely used, for applications ranging from vehicle propulsion to electrical power generation, and many in many others. All internal combustion engines operate based upon the same fundamental principle of igniting a charge of a combustible fuel with oxidant in a cylinder to produce a rapid pressure and temperature rise that drives a piston coupled with a rotatable crankshaft. Spark-ignited engines such as gasoline engines to power passenger cars and small power equipment, and diesel engines in both light duty and heavy duty vehicle, machinery and electric power generation applications will be familiar to most. In recent years, there has been increasing interest in the development of internal combustion engines that operate on alternative fuels, including gaseous fuels such as natural gas, biogas, landfill gas, and still others.
Modern engines tend to be highly sophisticated pieces of equipment, with numerous different systems and subsystems the operation of which must be monitored and frequently or continually adjusted to conform with various specifications as well as changing operational demands. An intake system conveys air and sometimes also fuel, and potentially recirculated exhaust gas, to a cylinder in the engine for combustion. The intake system can include filters, one or more compressors, coolers, and various items of monitoring equipment for enabling pressure and temperature at various locations in the intake system to be monitored and controlled. An exhaust system can include one or more turbines, particulate filters, catalysts, and other mechanisms for treating exhaust, and still other equipment. The fuel system stores fuel, typically pressurizes the fuel, and delivers the fuel by way of the intake system or, for example, by direct fuel injection, to the cylinders for combustion.
Regardless of engine type and associated engine equipment, it is typically desirable to control a rotational speed (RPM) of the engine to enable the engine to operate at a power output, an exhaust/emissions output, an efficiency or otherwise in a desired or specified manner. All of the above systems/subsystems, and others not mentioned, can be impacted by and/or affect engine speed control. While various engine speed control strategies have been proposed over the years, many engines can be classified generally as either “throttle governed” or “fuel governed.” In a fuel governed engine, an engine speed error, the difference between a desired engine speed and an actual or observed engine speed, is typically used to set a fuel flow command, and a throttle position is varied to provide a desired air-to-fuel ratio (AFR) based on the amount of fuel that is being requested. Liquid fueled engines, including diesel engines, some gaseous fuel engines, and some dual fuel engines can be fuel governed. In throttle governing strategies, engine speed error is used to set a desired intake or inlet manifold pressure (IMAP), and the throttle is adjusted in an attempt to attain the desired IMAP. Throttle governing is commonly applied to gaseous fuel engines. These and other strategies have their advantages and disadvantages, and there is always room for improvement and/or alternatives. An example engine speed control strategy is known from U.S. Pat. No. 6,021,755 to Maddock et al., in which a fuel command is apparently generated based on manifold air pressure and temperature, and the fuel command then modified on the basis of a comparison of desired and actual engine speeds.